HBV well-known as a Dane particle is a spherical particle having a diameter of about 42 nm and composed of an outer envelope and a nucleocapsid. The outer envelope surrounding the nucleocapsid contains a large quantity of hepatitis B surface antigens, and the nucleocapsid comprising about 180 subunits of hepatitis B core protein includes several kinds of genes encoding HBV structural proteins, polymerase and so on (Summers et al., Proc. Nat. Acad. Sci. 72: 4579, 1975; Pierre Tiollais et al., Science 213: 406-411, 1981).
The gene coding region of HBV surface antigens contains three in-frame initiation sites, and all these sites share a termination codon of S-domain terminal. Therefore, the HBV surface antigens can be divided into three groups: (i) a small hepatitis B virus surface antigen (hereinafter, referred to as “S-surface antigen”) containing only the S-domain; (ii) a middle hepatitis B virus surface antigen (hereinafter, referred to as “M-surface antigen”) containing the S-domain and Pre-S2 consisting of 55 amino acids; and (iii) a large hepatitis B virus surface antigen (hereinafter, referred to as “L-surface antigen”) containing the S-domain, Pre-S2 and Pre-S1. The S-surface antigen accounts for about 80% or more of the total expressed surface antigen.
Interferon α and Lamivudin have been widely used for treating chronic hepatitis (C. L. Lai and P. C. Wu, H.K.M.J. 3: 289-296, 1997). However, interferon α shows high toxicity in spite of its immune response against in vivo virus. On the other hand, Lamivudin acting as a nucleic acid derivative shows anti-viral activity by way of inhibiting DNA polymerase. A lamivudin formulation for oral administration exhibits high therapeutic effect, but when it is taken for a long time, there is a high risk of inducing a mutant resistant to Lamivudin (Daryl T. Y. Lau et al, Hepatology 32: 828-834, 2000). Further, an anti-HBV polyclonal antibody isolated from human serum has been used for preventing HBV infection from liver transplantation and vertical transmission. However, this antibody also has problems in that it shows low antigen specificity and must be isolated from human blood.
A method employing a mouse monoclonal antibody against HBV surface antigen has been recently developed so as to solve the problems mentioned above, but a human antibody is formed against the mouse monoclonal antibody when used for a long time (Dimaggio J. J., et al., Cancer Chemother. Biol. Response Modif 11: 177-203, 1990).
To overcome the undesirable properties of mouse monoclonal antibodies, a humanized antibody has been developed by replacing the framework regions except for the antigen-binding site with regions of a human antibody. A method currently used for preparing such a humanized antibody comprises the steps of selecting a gene encoding the human antibody showing the closest sequence similarity to the mouse antibody and replacing only the complementarity determining region (CDR) of the mouse antibody with that of the human antibody by way of a CDR grafting method. The humanized antibody has the advantage of reducing the in vivo immune response (Riechmann et al., Nature 332: 323, 1988; Nakatani et al., Protein Engineering 7: 435, 1994). However, when only the CDR is grafted on the human antibody, its selectivity and reactivity are often compromised (Carter P., et al., Proc. Natl. Acad. Sci. USA 89: 4285-4289, 1992).
The present inventors have endeavored to overcome such problems of the conventional humanized antibody, and developed a novel humanized antibody against S-surface antigen of HBV which maintains antibody specificity of a mouse monoclonal antibody against HBV S-surface antigen while minimizing the immune response to the mouse monoclonal antibody.